Fabrication and Properties of Carbon-Encapsulated Cobalt Nanoparticles over NaCl by CVD
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Carbon-encapsulated cobalt (Co@C) nanoparticles, with a tunable structure, were synthesized by chemical vapor deposition using Co nanoparticles as the catalyst and supported on a water-soluble substrate (sodium chloride), which was easily removed by washing and centrifugation. The influences of growth temperature and time on the structure and magnetic properties of the Co@C nanoparticles were systematically investigated. For different growth temperatures, the magnetic Co nanoparticles were encapsulated by different types of carbon layers, including amorphous carbon layers, graphitic layers, and carbon nanofibers. This inferred a close relationship between the structure of the carbon-encapsulated metal nanoparticles and the growth temperature. At a fixed growth temperature of 400 °C, prolonged growth time caused an increase in thickness of the carbon layers. The magnetic characterization indicated that the magnetic properties of the obtained Co@C nanoparticles depend not only on the graphitization but also on the thickness of the encapsulated carbon layer, which were easily controlled by the growth temperatures and times. Optimization of the synthesis process allowed achieving relatively high coercivity of the synthesized Co@C nanoparticles and enhancement of its ferromagnetic properties, which make this system promising as a magnetic material, particularly for high-density magnetic recording applications.
KeywordsCarbon-encapsulated metal nanoparticles Sodium chloride Chemical vapor deposition Magnetic property
Carbon-encapsulated metal nanoparticles
Chemical vapor deposition
Field emission scanning electron microscope
High-resolution transmission electron microscope
Magnetic metal nanoparticles (such as Co, Ni, and Fe) have attracted significant attention due to their enhanced electronic and magnetic properties. These properties have potential applications in ferrofluids, magnetic resonance imaging, and magnetic recording [1, 2, 3, 4]. However, there are a series of problems that restrict their wide applications, such as oxidation, agglomeration, and instability in air. Carbon-encapsulated metal nanoparticles (CEMNPs), which may address the above problems, have attracted increasing interest due to their unique core/shell structure and properties [5, 6]. The role of carbon-encapsulating layers is to isolate the metal nanoparticles from each other, thus avoiding the problems caused by interactions among closely compacted magnetic units. Furthermore, stability and compatibility of the metal nanoparticles in application environments are enhanced by these protective carbon layers [7, 8, 9]. CEMNPs are thus endowed with new potential applications in the fields of catalytic synthesis, biomedicine, high-density magnetic data storage, and ferrofluids [10, 11, 12].
Various preparation methods, including arc discharge, chemical vapor deposition (CVD), pyrolysis, and explosions [13, 14, 15, 16, 17], have been developed to synthesize CEMNPs. Among these, CVD technique has received a particular attention due to its advantages such as a relatively low cost, potentially high yield, availability of raw materials, and a simple synthesis process . However, there are still difficulties in achieving a controllable adjustment of magnetic properties of CEMNPs, particularly, due to a lack of knowledge on the relationship between the structure and magnetic property of CEMNPs. Additionally, ceramic materials such as Al2O3, SiC, and MgO are often used as the catalytic support to synthesize CEMNPs during the CVD process [19, 20]. However, the ceramic catalytic support creates additional problem because it is difficult to completely remove from the final products. This reduces the purity and affects the properties of CEMNPs. To solve this problem, utilization of a water-soluble material as a catalytic support instead of a ceramic one could be considered as a promising alternative. For example, Steigerwalt et al.  prepared graphitic carbon nanofibers with a catalyst supported by three different water-soluble compounds Na4SiO4, Na2SiO3, and Na2CO3. Zhao et al.  synthesized hierarchical porous carbon with a graphitic structure by a water-soluble template method. However, during the synthesis of CEMNPs by CVD method, a water-soluble support is seldom used because they usually have lower melting point and more unstable chemically compared with the ceramic supports. Therefore, it is still challenging to synthesize CEMNPs with high purity, a controllable structure, and good magnetic properties.
Preparation of Co@C Nanoparticles
NaCl powder (99.5 % purity) was added into 0.1 mol L−1 Co(NO3)2·6H2O (99.0 % purity) aqueous solution with constant stirring for 0.5 h, ensuring achieving a weight ratio of Co/NaCl remained at 1/98. The aqueous solution was dried at 60 °C for 72 h to acquire Co(NO3)2/NaCl mixture, which was then calcined in argon (99.99 % purity, 100 mL min−1) at 350 °C for 1 h, and the CoO/NaCl catalyst precursor was obtained. Synthesis of the Co@C nanoparticles included reducing the CoO/NaCl catalyst precursor in hydrogen gas (99.99 % purity, 100 mL min−1) at 350 °C for 2 h to acquire the Co/NaCl catalyst. The next step of the preparation process was a heat treatment of the sample in mixture gas of acetylene (99.9 % purity, 30 mL min−1)/argon (420 mL min−1) at different growth temperatures (350, 400, 450, and 500 °C) for 60 min. In addition, a similar synthesis process was performed at 400 °C for different growth times (5, 15, and 30 min). Black powders were obtained after the reactor was cooled to room temperature in an argon atmosphere (100 mL min−1).
Purification of Co@C Nanoparticles
To purify the Co@C nanoparticles, the prepared black powders were thoroughly washed by ultrasonication in distilled water in a round-bottomed flask for 1 h. The precipitate was separated from liquid phase using a high-speed centrifuge (Evolution RC; Thermo Electron LED GmbH) at 8000 rpm for 30 min. The resulting solid precipitates were dried in a dryer at 80 °C for 24 h, and the final Co@C nanoparticles were obtained.
Surface morphology and microstructure of the Co@C nanoparticles were characterized using field emission scanning electron microscope (FE-SEM; Hitachi S-4800) and high-resolution transmission electron microscope (HRTEM; Philips Tecnai G2 F20). X-ray diffraction (XRD) patterns of the Co@C nanoparticles were recorded on a powder X-ray diffractometer (Rigaku D/max 2000V/pc) using CuKα radiation within the angle range of 20°–90° (2θ) with a 0.02° step. The interplanar spacing of the graphite layer was calculated using the Digital Micrograph software. Raman spectroscopy was performed using the 532-nm line of Ar+ laser as the excitation source on a Thermo Fisher DXR Raman Microscope. Thermal properties of the samples were investigated using thermogravimetric analysis (TGA, TA Instruments SDT Q600 TGA). The samples in TGA were analyzed in platinum pans at a heating rate of 10 °C min−1 from 50 to 700 °C in air with a flow rate of 150 mL min−1. The magnetic properties of the samples were measured using a vibrating sample magnetometer (VSM; Lakeshore 7407) at room temperature.
Results and Discussion
Magnetic properties of pure Co and Co@C nanoparticles grown at different temperatures for 60 min
M s (emu g−1)
M r (emu g−1)
M r/M s
H c (Oe)
Co@C nanoparticles grown at 350 °C
Co@C nanoparticles grown at 400 °C
Co@C nanoparticles grown at 450 °C
Co@C nanoparticles and CNFs grown at 500 °C
Magnetic properties of Co@C nanoparticles grown at 400 °C for different times
M s (emu g−1)
M r (emu g−1)
M r/M s
H c (Oe)
Co@C nanoparticles grown at 400 °C for 5 min
Co@C nanoparticles grown at 400 °C for 15 min
Co@C nanoparticles grown at 400 °C for 30 min
The Co@C nanoparticles were synthesized using NaCl as the catalytic support at different growth temperatures and times using the CVD method. By using NaCl as the catalytic support, the purification of the Co@C nanoparticles is easier and consists of only simple washing and centrifugation. At low growth temperatures (350 °C), the Co nanoparticles are encapsulated in amorphous carbon shells, which have a lower graphitization degree and oxidation stability. At 400 and 450 °C, the Co@C nanoparticles with well-ordered graphitic shells and good thermal oxidation stability are obtained. A higher growth temperature (500 °C) reduces the purity of the Co@C nanoparticles due to the formation of CNFs. The growth time is also a key factor in controlling the thickness of carbon layers. The magnetic characterization shows that the grown Co@C nanoparticles possess relatively large coercive force and good ferromagnetism, and the magnetic properties are closely related to their structures. Thus, by controlling the growth temperature and time of CVD in this process, the purity, morphology, structure, and magnetic property of Co@C nanoparticles could be easily tuned. Meanwhile, the high coercivity of the Co@C nanoparticles enabled the material to bear a larger degree of demagnetization, which indicates their great application potential in the field of high-density magnetic recording materials.
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51201056, 21406052), Scientific Research Foundation for Selected Overseas Chinese Scholars, Ministry of Human Resources and Social Security of China (Grant No. CG2015003002), the Program for the Outstanding Young Talents of Hebei Province (Grant No. BJ2014010), and the Natural Science Foundation of Hebei Province of China (Project No. E2015202037, E2013202021). ZB acknowledges the research grant 4649/GF4 and 5156/GF4 from the Ministry of Education and Science of Kazakhstan.
HL and YGZ conceived and designed the experiments. YL, HW, and BL carried out the experiments. HL and CL analyzed the data. HL, YGZ, DA, and ZB contributed to the drafting and revision of the manuscript. HL and YGZ supervised the work and finalized the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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